Wolf 940

Larger composite image (where
the brown dwarf "b" is bluish,
here, in infrared but would
appear magenta in visible light)

Wolf 940 is a red dwarf star
with a very cool, methane
brown dwarf companion in
a wide orbit
(more).

System Summary

Wolf 940 is located
about 39.9 light-years away in the north central part
(21:46:40.5-0:10:25.4, ICRS 2000.0) of Constellation
Aquarius,
the Waterbearer -- southwest of
Sadalmelik
(Alpha Aquarii); east of Globular Cluster
M2;
northeast of
Sadalsuud
(Beta Aquarii); south of
Enif
(Epsilon Pegasi) and north of
Deneb
Algedi (Delta Capricorni). The relatively high proper motion of
Wolf 940 was probably first discovered in the early 20th Century by
Max
(Maximilian Franz Joseph Cornelius) Wolf (1863-1932), a
pioneer of astrophotography who discovered hundreds of variable
stars and asteroids, and about 5,000 nebulae by analyzing
photographic plates and developing the "dry plate" in 1880 and
the "blink comparator" in 1900 with the Carl Zeiss optics
company in Jena, Germany. On April 20, 2009, a team of astronomers
announced that they had imaged a cool methane brown dwarf companion
in a wide orbit around Wolf 940 (UKIRT/JAC
press
release; and Gemini
press release --
more below). (See an animation of the
possible orbits of Wolf 940, its
potentially habitable zone, and its substellar companion, with
a table of basic orbital and physical characteristics.)

On April 20, 2009, a team of astronomers announced that
they had imaged a cool methane brown dwarf companion "b"
in a wide orbit around Wolf 940, using the
United
Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii
as part of the
UKIRT Infrared Deep Sky Survey
(UKIRT/JAC
press
release; and Gemini
press release).
One of the coolest known brown dwarfs detected as of April 20, 2009,
the object has about 20 to 30 times Jupiter's mass, roughly the same
diameter, and a surface temperature of around 300 °C.
Although Wolf 940 b glows brightly in infrared wavelengths, it does
not emit much visible light because of its low surface temperature.
Like Jupiter and the methane brown dwarf
Gliese 229 b it has an
abundance of methane on its surface and has been given a spectral
type of T8.5. The object is currently separated from Wolf 940 by
about 440 AUs (beyond the observed orbital distance of
Edgeworth-Kuper Belt objects in the
Solar System), and an orbital period of around 18,000 years.
(See an animation of the
possible orbits of Wolf 940, its
potentially habitable zone, and its substellar companion, with
a table of basic orbital and physical characteristics.)

When brown dwarfs were just a theoretical concern, astronomers
differentiated those hypothetical objects from planets by how they were
formed. If a substellar object was formed the way a star does, from a
collapsing cloud of interstellar gas and dust, then it would be called
a brown dwarf. If it was formed by gradually accumulating gas and dust
inside a star's circumstellar disk, however, it was called a planet.
Once the first brown dwarf candidates were actually found, however,
astronomers realized that it was actually quite difficult to definitely
rule on the validity of competing hypotheses about how a substellar
object was actually formed without having been there. This problem is
particularly difficult to resolve in the case of stellar companions,
objects that orbit a star -- or two.

Although brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to
ignite core hydrogen fusion, the smallest true stars (red dwarfs) can
have such cool atmospheric temperatures (below 4,000° K) that it is
difficult to distinguish them from brown dwarfs. While
Jupiter-class planets
may be much less massive than brown dwarfs, they are about the same
diameter and may contain many of the same atmospheric molecules.

University of California at Berkeley astronomer
Ben R. Oppenheimer, who
helped to discover Gliese 229 b, is part of a growing group that would
like to define a brown dwarf as an substellar object with the mass of
13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary"
hydrogen (a single proton nucleus) like stars, they have enough mass to
briefly fuse deuterium (hydrogen with a proton-neutron nucleus).
Therefore, stellar companions with less than 13 Jupiter masses would be
defined as planets.

Other prominent astronomers, such as San Francisco State University
astronomer Geoffrey W. Marcy
who also has helped to discover many extrasolar planets, note that there may
in fact be many different physical processes that lead to the formation
of planets. Similarly, there may also be many different processes that
lead to the creation of brown dwarfs, and some of these may also lead
to planets. Hence, more observational data may be needed before
astronomers can determine how to make justifiable distinctions in the
classification of such substellar objects. (More information on this
debate over definitions is available at
exoplanets.org.)

In visible light at one billion years in age,
large brown dwarfs are reddish like the smallest
M-type stars, but cooler, dimmer T-dwarfs are
more magenta in hue. At least 13 brown dwarfs
may be located within 10 parsecs of Sol
(more).

Cool Methane Brown Dwarfs

While brown dwarfs have too little mass to fuse "regular" hydrogen
(which has a single proton nucleus), virtually all of the ones
discovered until 1999 were too hot -- that is "young" -- to show
evidence of methane which is destroyed by stellar temperatures. While
methane is a atmospheric characteristic of giant gas planets like
Jupiter, the first brown dwarf found to have a trace of methane was
Gliese 229 b.

In Spring 1999, two very dim and reddish brown dwarfs were
found as solitary objects (one 30 light-years away in Ophiuchus and
another also relatively nearby in Virgo). Analysis of their spectra
indicated that both have atmospheres that are rich in methane. In
addition, four similar objects that are too cool to be observed in
visible light were found using near-infrared telescopes also to have
the methane fingerprint of extremely cool (that is "old") brown dwarfs.
These discoveries provided strong evidence that, although hard for
astronomers to detect, faint brown dwarfs which have had billions of
years to cool may represent a significant population of the universe.
Astronomers then began speculating that these objects may well be as
numerous as the stars, reviving theories of stellar formation that
suggest the existence of uncountably numerous brown dwarfs, rather
than the relatively few easy-to-detect, bright ones found thus far.
By May 1, 2009, a list of
T
dwarfs compiled at
DwarfArchives.org
indicated that at least 155 methane brown dwarfs had been detected
and confirmed over the past decade.

Closest Neighbors

The following star systems are located within 10
light-years, plus more bright stars within 10 to
20 ly, of Wolf 940.

Aquarius was "Latinized" by the Romans from Ganymede in Greek mythology,
who was "cup-bearer to the gods."
For more information on stars and other objects in Constellation
Aquarius and an illustration, go to Christine Kronberg's
Aquarius.
For another illustration, see David Haworth's
Aquarius.

For more information about stars including spectral and luminosity
class codes, go to ChView's webpage on
The Stars of
the Milky Way.